Field of the Invention
Synchronous Digital Hierarchy (SDH) equipment is the latest generation of equipment that is used to provide high bandwidth communications capabilities for use between telephone exchanges and in other areas where high quality telecomms is required (broadcast video distribution, etc). Embedded within the traffic xe2x80x98trafficxe2x80x99 carrying capability of the equipment are data communications channels (DCCs). These channels constitute a datacomms network that uses OSI protocols.
Each piece of equipment constitutes a routeing node in the datacomms network formed by the data channels, and can operate any one of a number of different routeing methods. The present invention is concerned with the interworking of two of the possible routeing methods.
The two routeing methods that will commonly occur in SDH networks are ISxe2x80x94IS (ISO 10589) and quasi-static routeing (where alternate routes may be chosen on link failure). Where this occurs, routeing loops, causing loss of communications can be caused. The present invention detects the formation of a routeing loop and changes the behavior of the ISxe2x80x94IS node accordingly.
The ISxe2x80x94IS routeing protocol is one of a set of xe2x80x98link statexe2x80x99 dynamic routeing protocols. These protocols automatically distribute routeing information round the datacomms network, allowing nodes to learn the required routeing information from the actual network. This provides the ability to automatically reconfigure, allowing routeing round network faults, in case of network link failure.
The ISxe2x80x94IS routeing protocol has two routeing levels, Level-1 and Level-2. See FIG. 2 (from ISO 10589) for the use of these levels and the general environment of this protocol and the topologies and systems supported by Intradomain Routeing.
The present invention is also applicable to other datacomms scenarios, where a dynamic routeing protocol is interworked with static routeing, or a different dynamic protocol (e.g. OSPF and static routes, etc).
According to the present invention there is provided a Synchronous Digital Hierarchy (SDH) based communications network comprising a plurality of Intermediate Systems (IS), the ISs being divided between at least one ISxe2x80x94IS Area and at least one non-ISxe2x80x94IS Area, an ISxe2x80x94IS Area being an area with which a routeing protocol forming part of the Network Layer (Layer 3) of the Open Systems Interconnection including routeing (OIS), is provided for routeing messages between areas, including routeing means, whereby where a message is routed from an ISxe2x80x94IS Area to a destination IS within a non-ISxe2x80x94IS Area and the connection to the destination IS is broken, and as a result a message is returned from the non-ISxe2x80x94IS Area to the originating ISxe2x80x94IS Area connection to the destination IS is made by a second choice connection.
There is further provided a method for use in Synchronous Digital Hierarchy (SDH) based communications network comprising a plurality of Intermediate Systems (IS), the IS being divided between at least one ISxe2x80x94IS Area and at least one non-ISxe2x80x94IS Area, wherein when a message is returned from the non-ISxe2x80x94IS Area to the originating ISxe2x80x94IS Area a second choice connection to the destination IS is made.